The bacterial reaction center (RC) is a membrane bound protein that plays the central role in the conversion lectromagnetic radiation into chemical energy. The RC catalyzes the light driven electron transfer from cytochrome c2 to a quinone-Fe complex, that contains a primary quinone QA and a secondary quinone QB. The primary quinone QA, plays an important role in the initial electron transfer step. The secondary quinone QB plays a central role in the electron and proteon translocation process, acting as a two electron two proton acceptor species which is involved in the primary step of proton translocation. This proposal addresses the question of the molecular basis for the function of the quinones using the technique of site directed mutagenesis in the bacterium Rb. sphaeroides. By modifying amino acid residues near the Q binding sites and measuring the concomitant changes in activity we will identify the residues involved in the following functions: a) Quinone and herbicide binding. Residues near the quinone binding sites will be modified and the binding constants of quinone and herbicide will be determined. The results will give information about the nature of substrate and inhibitor binding and the mechanism of herbicide resistance. b) Electron transfer kinetics. Aromatic and charged amino acids will be modified and their effects on electron tyransfer determined. The results should give information about the mechanisms of electron transfer. c) Electrostatic stabilization of QQ -. Charged amino acid residues will be modified and the equilibrium between Q-A QB and QAQ-B will be determined. The results should give information about the electrostatic interactions in a membrane protein. d) Proton transport to QB. Protonable residues which can potentialy form a proton transfer chain from the RC surface to the QB site will be modified and their effect on the rate of proton transfer will be detemined. The results should test the mechanism of proton transport in biological membranes.